1. Field of the Invention
The present invention relates to techniques of locally removing the thin film layer covering the surface of a substrate such as a semiconductor wafer, and more particularly to a method and apparatus whereby in order to improve the performance of lithographic apparatus (particularly, exposure apparatus) used in the manufacture of semiconductor integrated circuits, the thin film layer of a photosensitive material or the like covering the surface of a substrate preliminarily formed with patterns such as ICs is locally removed.
2. Description of the Prior Art
With the recent trend toward increasing the level of integration of VLSI patterns, the minimum line width of circuits has been decreased. It is expected that an excimer stepper using a deep ultra violet (UV) light source, e.g., a KrF (fluosilicate krypton) excimer laser having a wavelength of 248 nm as a light source will be mainly used for the manufacture of the next-generation memories whose minimum line width is on the order of 0.35 .mu.m on semiconductor chips or 64 Mbit D-RAMs (Dynamic Random Access Memories).
With this type of excimer stepper, it is desirable that an alignment measurement (mark detection) of the through-the-lens (TTL) system is effected through a reticle serving as a projection negative in order to improve the accuracy of the alignment (particularly, the wafer alignment).
In the case of this TTL system through a reticle, it is not possible to make an excellent mark position measurement unless light of the same wavelength as the exposure light is used as an illuminating light for detecting the alignment marks on a wafer.
The reason is that the projection lens system used in the excimer syepper has an intense chromatic aberration so that the image forming conjugatory relation between the reticle and the wafer cannot be maintained by use of a non-exposure light such as a visible light.
In view of these circumstances, it has been conceived to use an excimer laser light divided from the same light source as the exposure light as an illuminating light for the TTL-type alignment of the excimer stepper. Where the excimer laser light is used for detecting the marks on a wafer, however, there are many cases in which the excimer laser light is intensely absorbed by the resist layer uniformly applied onto the wafer. In the case of a novolac-type photoresist, e.g., MP-2400 (the tradename of Shipley Co.), for example, the photoresist layer applied to a thickness of 1 .mu.m has a transmittance of as low as 5 to 8% and also the quantity of the light reflected from the mark and returned to the projection lens system represents a double-trip transmittance through the reisist layer thus allowing to expect only the transmittance of about 0.64% at the most.
Such a low transmittance can never permit the alignment measurement so that even if the mark detection itself is possible, its position measuring accuracy and repeatability are deteriorated extremely.
Thus, as a measure to counter it, it has been conceived to locally remove the resist layer only on the alignment mark portions prior to the alignment operation.
FIGS. 17A and 17B show a conventional method in which the resist layer on the mark portions is removed through the exposure and development due to the projection of a light beam.
FIG. 17A shows the manner in which a resist removing apparatus is incorporated in a stepper. In this case, the stepper comprises a mercury lamp HG, a condenser lens CL, a reticle R, a projection lens PL and a stage ST for moving a wafer W in x and y directions in a step-and-repeat mode, and the resist removing apparatus includes a composite tube assembly TB.
FIG. 17B shows the construction of one forward end of the tube assembly TB, and in this case the tube assembly TB includes an outer tube OP, an inner tube IP coaxially inserted into the outer tube OP with a gap formed therebetween, an optical fiber FB inserted into the inner tube IP with a gap therebetween and having a lens GL at the forward end, and an annular packing PK provided at the forward end face of the outer tube OP.
Formed on the surface of the wafer W as shown in FIG. 17B are alignment marks M so that a coating AL to be photo etched is formed on the marks M and a resist layer Pr is applied onto the top surface. The tube assembly TB is vertically movable in the condition of FIG. 17A so that when the resist layer Pr is to be removed, the tube assembly TB is lowered so as to be pressed against the resist layer Pr through the packings PK as shown in FIG. 17B.
Then, where the resist layer Pr is the positive type, an exposure light IL is projected onto each mark M from the lens GL through the optical fiber FB. Then, a developing solution is supplied to the resist layer Pr through inside the inner tube IP and the developing solution is discharged toward an absorbing unit which is not shown through the space between the inner tube IP and the outer tube OP. Then, a rinsing solution is supplied through the inner tube IP thereby rinsing the developed portion enclosed by the packing PK. Then, N.sub.2 (nitrogen) gas is supplied from the inner tube IP thereby effecting the drying. When the removing of the resist is completed in this way, the tube assembly TB is moved upward to separate from the wafer W and then the ordinary alignment operation and exposure operation are initiated.
It is to be noted that in order to locally remove the resist layer, in addition to the above-mentioned photolithographic method, a photo etching method requiring no post developing operation may be used so that in this case a high-energy ultraviolet light beam such as an excimer laser beam is projected onto the resist layer Pr near the mark M to break the molecular bond of the resist and the vaporized and scattered molecules and the remaining fine particles are purged and discharged while supplying N.sub.2 gas from the inner tube IP.
In the above-mentioned conventional method, the component part for removing the resist layer Pr (the packing PK at the forward end of the tube TB) is pressed against the surface of the wafer and thus there is the great danger of causing defects (flaws, etc.,) on the resist layer, thereby making it impossible to put the method as such into practical use.
Moreover, in accordance with the conventional method the component part for removing the resistor layer Pr is pressed against the wafer surface to remove the resist so that during the interval the cleanliness of the wafer surface cannot be maintained and also the resist removal at an economically adequate rate cannot be effected.
A conventional apparatus requires that a wafer is mounted in a prealigned condition on an X-Y stage ST and for this purpose it is necessary to provide a prealignment station based on the external shape of wafers and a transfer unit (e.g., a loader arm) for the X-Y stage ST, thus inevitably decreasing the processing speed. Also, since the X-Y stage ST is used for the positioning of a part (the tubes TB) of the removing apparatus relative to the localized portions (the mark portions) to be removed on the resist layer, the operations for moving and positioning the stage are required and the throughput is inevitably deteriorated in cases involving a large number of portions to be removed.